Elsevier

Surface Science

Volume 632, February 2015, Pages 98-102
Surface Science

Local atomic configuration of graphene, buffer layer, and precursor layer on SiC(0001) by photoelectron diffraction

https://doi.org/10.1016/j.susc.2014.09.021Get rights and content

Highlights

  • Full hemispherical photoelectron diffraction patterns were measured for local sites selectively.

  • Each graphene formation stage on the SiC by thermal process was characterized.

  • The bonding angles for the graphene buffer and the precursor layers were distributed within the polar angle range of 75–90°.

  • The local atomic configurations for both precursor and buffer layers were shown to be basically similar.

Abstract

The layer-specific atomic structure of the 4H-SiC(0001) surface at each stage of single-layer graphene formation was studied using photoelectron diffraction. The 2π-sr C 1s photoelectron intensity angular distributions (PIADs) excited by circularly-polarized soft X-ray were measured. Taking the photoelectron inelastic mean free path into account, we have separated C 1s PIADs of the graphene overlayer together with the interface buffer layer beneath it and the precursor layer, (6√3 × 6√3)-R30°, from that of the SiC substrate. Clear diffraction rings due to the intra-layer Csingle bondC bond scattering of graphene were observed. The forward focusing peaks indicating the directions of the neighboring atoms seen from the C atoms directly bonded to the SiC substrate were observed for the precursor and interface buffer layers. This result suggests that the structures of both layers have a common local atomic configuration.

Introduction

Graphene is a single-layer carbon network arranged in a honeycomb lattice, and is expected to be a useful material for next-generation devices [1]. Epitaxial growth by the thermal decomposition of SiC substrate is a promising method for fabricating high-quality uniform graphene. To clarify the mechanism of graphene formation and determine the surface atomic arrangements at each stage, various kinds of surface analyses have been reported. Early studies revealed that the SiC(0001) surface at the graphitization precursor stage is terminated by a graphite-like structure covalently bonded to the SiC substrate with -(6√3 × 6√3)-R30° periodicity (hereinafter, called the precursor layer) [2], [3], [4], [5]. Further elimination of Si leads to the formation of single-layer graphene (SLG) on a C-rich buffer layer [4], [5], [6], [7]. Interaction of the buffer layer with graphene as well as with the substrate greatly affects the electronic properties of graphene [8]. Characterization of the buried buffer layer is a key to improving the performance of graphene devices. However, the atomic structure of the subsurface of SLG remains unclear.

A photoelectron from a localized core level is an excellent element-selective probe for surface structure analysis [8]. Forward focusing peaks (FFPs) appearing in the photoelectron intensity angular distribution (PIAD) indicate the directions of surrounding atoms as seen from the photoelectron emitter atom [9]. The distance between the emitter and scatterer atoms can be deduced from the circular dichroism shift of the FFP position [9], [10], [11] as well as from the opening angles of diffraction rings (DRs) appearing around the FFP [11], [12]. Roth et al. used photoelectron diffraction (PED) to determine the structure of a deposition of a nearly free-standing graphene top layer and a corrugated boron nitride subsurface layer on a Cu(111) surface [13]. We have demonstrated that the stacking orientations of exfoliated bilayer graphene flakes can be determined by using PED [14].

Recently, de Lima et al. measured and separated C 1s PIADs of graphene and precursor layers on SiC(0001) by a chemical-specific PED approach [15]. They evaluated the ripple amplitude of long-range order in precursor layers and suggested just one type of hybridization for the local buckling structure. Furthermore, they proposed a rather flat structural model for the buffer layer. In fact, the buffer layer consists of various types of hybridizations, namely, sp2, sp3 and the intermediates [16], [17], [18]. However, their detection range did not include any FFP, which is the most sensitive signal for local structure.

Differentiating a series of PED data at the subsequent graphitization stages has been reported as an alternative approach for layer-resolved analysis. Matsui et al. successfully carried out an atomic-layer-resolved analysis of thin films of Ni by differentiating a series of PED data from wedged film [19]. Maejima et al. discussed the local subsurface structures of a thin SiON film grown on a SiC substrate [20]. In the current study, we have characterized each stage of the thermal graphitization of a 4H-SiC(0001) substrate by a similar approach. Site-specific PIADs were deduced by considering the dependence of escape depth on emission angle, and subtracting the substrate signal intensity. The local atomic arrangements of the SLG as well as buffer and precursor layers of the thermally graphitized substrate were investigated by layer-resolved photoelectron diffraction.

Section snippets

Experimental methods

Catalyst-referred etching (CARE) single-crystalline 4H-SiC(0001) wafer (N-doped, 0.02 Ω cm) with an on-axis oriented Si face (0 ± 0.5° off) was used as the substrate [21], [22]. The wafer has an atomically flat and damage-free surface with a terrace width of 300–500 nm. Hattori et al. reported a procedure to obtain a large scale uniform graphene by annealing this CARE-SiC substrate in ultrahigh vacuum (UHV) without using the Si flux method during thermal epitaxial growth [23]. This specially treated

Results and discussion

The chemical composition at each stage of graphitization was characterized by constant-final-state-mode two-dimensional photoelectron spectroscopy. The photoelectron kinetic energy was fixed at 800 eV, while the photon energy was scanned from 800 eV to 1160 eV, so that we can detect the C 1s and Si 2p core-levels at the same kinetic energy. In this way, the probing depth of the photoelectron for each emission angle was kept constant for different binding energies.

Fig. 1 shows the signal intensity

Conclusion

In conclusion, we have characterized each stage of thermal graphitization of the 4H-SiC(0001) substrate by using PED. PIADs of the precursor layer and SLG with buffer layer were deduced by considering the dependence of escape depth on emission angle and subtracting the SiC substrate signal intensity. The photoelectron diffraction analysis made it possible to directly determine the local structure of the surface and interface.

Acknowledgments

This work was performed with the approval of the Japan Synchrotron Radiation Institute, Proposal No. 2009B1769, 2010A1469, and 2011A1471. The authors express deepest gratitude to Dr. Tetsuya Nakamura and Dr. Takayuki Muro for their experimental supports. This study was also supported by a Grant-in-Aid for JSPS Fellows, No. 11J09374.

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